This invention relates to bonded sandwich structures used for structural applications, and, more particularly, to the reduction in weight of such structures without loss of mechanical properties.
Adhesively bonded sandwich structures are widely used in applications such as aircraft and spacecraft, where high stiffness is required with minimal weight. In such a structure, face sheets, typically aluminum or composite material panels, are joined to each side of a core, typically metallic or nonmetallic honeycomb. The honeycomb holds the face sheets at the required positions. This structural element provides high panel stiffness and strength with minimal weight.
A key to the construction of the adhesively bonded sandwich structure is the method of joining the face sheets to the core. An adhesive is placed between the face sheet and the core, the face sheet is pressed to the core, and the adhesive is cured. However, if the adhesive is applied in bulk, the amount applied is usually in excess of that required, resulting in the addition of unproductive weight to the structure. To apply exactly the right amount of adhesive in a convenient form, film adhesives have been developed and are commercially available. The film adhesive is a curable adhesive sheet, typically about 0.003 inch thick and weighing on the order of 0.02 pounds per square foot. To construct a panel using a film adhesive, the film adhesive is placed between the core and the face sheet, the array is pressed together, and the adhesive is cured.
This film adhesive approach works well, is convenient to use, reduces the weight of adhesive in the structure to the thickness required, and is widely used in the industry. However, it has been recognized that some of the adhesive applied by this approach is not effective in bonding the structure. Specifically, that portion of the adhesive which does not contact the exposed honeycomb walls to which the face sheet is bonded adds weight but not strength to the structure.
It has been proposed to selectively perforate the film adhesive to reduce its weight. This approach is not satisfactory for use in many manufacturing operations, because a slight misalignment of the perforations and the honeycomb results in unbonded areas of the face sheet. Perfect alignment of hundreds of tiny perforations with the honeycomb cells is not possible practically. It is also known to preferentially position the adhesive at node locations in the honeycomb, but such approaches do not reduce the weight of the sandwich.
There is a need for a commercially practical approach for reducing the weight of adhesively bonded honeycomb and similar types of structures. The current invention fulfills this need, and further provides related advantages.